Silicon photonic hybrid polarization demultiplexer

ABSTRACT

An optical demultiplexer that includes at least one a hybrid phase shifter configured to receive a light signal over a fiber element, the light signal including polarized optical signals. Each phase shifter includes a thermo-optic phase shifter configured to phase shift the light signal, an electro-optic phase shifter configured to phase shift the light signal, and a coupler configured to maintain polarization of the polarized signal components. The optical demultiplexer also includes control circuitry configured to regulate the thermo-optic and electro-optic phase shifters.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/326,309 filed on Jul. 8, 2014, the contents of which is incorporatedby reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to an apparatus and method for providinga low-loss silicon photonic polarization demultiplexer having ahigh-bandwidth control loop.

BACKGROUND

Silicon photonics is an evolving technology that transmits data as lightpulses along optical fibers. Multiplexers combine the light pulses intoa single signal that is transmitted along an optical fiber where ademultiplexer divides the signal back into separate channels. Althoughactive demultiplexing exists in silicon photonics systems, the trackingspeed is far too slow to be of practical use. Tracking speed is limitedby the bandwidth, i.e., the response time, exhibited by state of the artdemultiplexers. Further, other types of demultiplexers exhibit largeinsertion loss that has proven to be unacceptable. Therefore, there is aneed for a demultiplexer that provides both sufficient control bandwidthand low insertion loss and that is applicable for silicon photonicssystems.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferredit being understood that the disclosure is not limited to thearrangements and instrumentalities shown, wherein:

FIG. 1 illustrates is an example of a silicon photonics networkutilizing the silicon photonic hybrid polarization demultiplexer of thepresent disclosure;

FIG. 2 illustrates an example of a hybrid phase shifter of the presentdisclosure using both thermal and electro-optic phase shifters;

FIG. 3 illustrates an example of the hybrid polarization demultiplexerof the present disclosure including a series of mixing stages; and

FIG. 4 illustrates a method incorporating the silicon photonic hybridpolarization demultiplexer of the present disclosure.

DESCRIPTION OF EXAMPLE EMBODIMENTS

The detailed description set forth below is intended as a description ofvarious configurations of the subject technology and is not intended torepresent the only configurations in which the subject technology can bepracticed. The appended drawings are incorporated herein and constitutea part of the detailed description. The detailed description includesspecific details for the purpose of providing a more thoroughunderstanding of the subject technology. However, it will be clear andapparent that the subject technology is not limited to the specificdetails set forth herein and may be practiced without these details. Insome instances, structures and components are shown in block diagramform in order to avoid obscuring the concepts of the subject technology.

Overview

In one aspect of the present disclosure, a polarization demultiplexer isprovided. The demultiplexer includes at least one a hybrid phase shifterconfigured to receive a light signal over a fiber element, the lightsignal including polarized optical signals. Each phase shifter includesa thermo-optic phase shifter configured to phase shift the polarizedoptical signal, an electro-optic phase shifter configured to phase shiftthe polarized optical signal, and control circuitry configured toregulate the thermo-optic and electro-optic phase shifters.

In another aspect of the present disclosure, a hybrid phase shifter isprovided. The hybrid phase shifter includes a thermo-optic phase shifterconfigured to phase shift polarized optical signals of a light signal,where the thermo-optic phase shifter receives a control signal fromcontrol circuitry. The hybrid phase shifter also includes anelectro-optic phase shifter configured to phase shift the polarizedoptical signals, where the electro-optic phase shifter receives a dithersignal from the control circuitry, the control signal and the dithersignal having different amplitudes.

In yet another aspect of the present disclosure, a method ofdemultiplexing a polarization-multiplexed optical signal is provided.The method includes receiving a light signal over a fiber element, thelight signal including polarized optical signals, and separating thepolarized optical signals of the light signal received over the fiberelement. The separating includes providing a plurality of mixing stages,each mixing stage comprising a thermo-optic phase shifter and anelectro-optic phase shifter adapted to phase shift the polarized opticalsignals.

DETAILED DESCRIPTION

The present disclosure describes a low-loss active polarizationdemultiplexer using pre-existing silicon photonics building blocks.Specifically, a hybrid polarization multiplexer that combines bothelectro-optic and thermo-optic phase shift elements is used in anoptical network. An example of an optical network 100 utilizing thehybrid demultiplexer of the present disclosure is shown in FIG. 1.Optical network 100 includes a transmitter 102, which includes a signalgenerator 104, having a laser 106 and a modulator 108, and a multiplexer110. Laser 106 creates a beam of light having light pulses 107 ofdifferent wavelengths. Modulator 108 encodes data onto each light pulse107. The modulated light is then passed on to multiplexer 110, whichcombines each of the multiple wavelengths of the modulated light onto aglass fiber 112, and on to receiver 119. Receiver 119 includes hybriddemultiplexer 114 of the present disclosure, which splits off theindividual wavelengths where they are sent to photodetectors 115, orcoherent receivers, that convert the optical data into electrical data.Logic 118 then processes the data. The optical network 100 of FIG. 1 isillustrative only, and the hybrid demultiplexer 114 of the presentdisclosure is equally applicable to other optical network designs.

FIG. 2 is an example of a hybrid phase shifter 121 that is one of thecomponents of hybrid polarization demultiplexer 114. Hybrid phaseshifter 121 includes both a thermal phase shifter 122 and anelectro-optic phase shifter 124. The combination of thermal phaseshifter 122 and electro-optic phase shifter 124 provides advantages thatdemultiplexers using only thermal phase shifters or only electro-opticphase shifter do not provide. Typical thermo-optic phase shiftersrequire a control bandwidth of approximately 10 kHz. This requiresdither tones to be applied at frequencies much larger than 10 kHz, whichis insufficient for implementation of a control loop. Thus, the use ofthermo-optic phase shifters alone cannot provide the adequate bandwidthnecessary for implementation of a control loop. On the other hand, theuse of only electro-optic phase shifters, due to their high loss,typically over 2 dB for a π phase shift, result in an unacceptableinsertion loss. In other words, the bandwidth of the thermo-optic phaseshifter is insufficient to support a dither signal and the electro-opticphase shifter has too high of an optical insertion loss to be effective.Advantageously, the use of hybrid elements, e.g., thermal phase shifter122 and electro-optic phase shifter 124, results in a high phaseexcursion, low loss, and sufficient bandwidth. A coupler 126, such as,for example a 2×2 coupler, can be used in connection with hybrid phaseshifter 121, to form a mixing stage 120, that maintains the polarizationof the multiplexed signal.

FIG. 3 illustrates a series of cascaded mixing stages 120, formingdemultiplexer 114, as part of a polarization multiplexed network. Eachmixing stage 120 includes hybrid phase shifter 121, which includesthermo phase shifter 122, electro-optic phase shifter 124, and coupler126. In the illustrated example, four mixing stages 120 are cascaded inorder to effectively separate the polarized signals of the light beam,as described above. Although four cascaded mixing stages 120 are shownin FIG. 3, it is within the spirit of the invention to include less ormore than the number of mixing stages 120 shown in FIG. 3. In theexample shown in FIG. 3, a light signal, comprised of two polarizedoptical signals, X and Y, are transmitted along a fiber. The fiberrotates and mixes these signals. A polarization splitter 128 separatesthe light signal into an X′ signal and a Y′ signal, each of whichrepresents a rotated and/or mixed version of the transmitted X and Ypolarized signals. Polarization demultiplexer 114 of the presentdisclosure is needed to recover the original signals X and Y from themixed X′ and Y′ signals. This is accomplished by application ofappropriate control voltages or currents, as discussed in further detailbelow.

Feedback control logic 130 applies the appropriate voltages tocontinuously regulate and adjust the signal applied to each mixing stage120. Feedback control logic 130 regulates each mixing stage 120 byapplying separate control and dither signals, rather than a combinedsignal. Error signals for feedback control logic 130 can be generatedfrom a variety of sources. For example, in one example, pilot tones orpilot signals are applied on one or both polarizations at transmitter102. In another example, an error signal can be generated from a biterror rate (“BER”) measurement.

Specifically, feedback control logic 130 applies control signals 132 toeach thermal-optic shifter 122 and separate dither signals 134 to eachelectro-optic shifter 124. In order to avoid interference betweencontrol signal 132 and dither signal 134, each dither signal 134 has asmaller amplitude than the amplitude of each control signals 132. In oneexample, dither signals 134 are on the order of a few percent of thecontrol signals 132. Thus, in this example, control signals 132 have abandwidth on the order of 10 s of kHz while dither signals 134 have abandwidth in the MHz range. Therefore, the length of each electro-opticphase shifter is relatively short, for example, on the order of 10microns or less, in comparison to the length of the thermal phaseshifter 122. FIGS. 2 and 3 illustrate this relative difference in phaseshift by depicting a longer thermal phase shifter 122 and a shorterelectro-optic phase shifter 124.

In the example shown in FIG. 3, where four mixing stages 120 areprovided in a cascaded fashion, if electro-optic phase elements alonewere used, the insertion loss would be approximately 8 dB (four stagesat a 2 dB loss per stage). If the polarization demultiplexer is basedsolely on thermo-optic phase shift elements, the insertion loss would beapproximately less than 0.5 dB but would result in insufficientbandwidth to allow practical dither-based control algorithms. However,because the high frequency dither signal used in hybrid demultiplexer114 of the present disclosure has a relatively small amplitude, thephase shift of the electro-optic phase shifter 124 is short, on theorder of 10 s of microns, and the resulting insertion loss is less than0.2 dB for each electro-optic phase shifter 124. Thermo-optic phaseshifters 122 can be sized to obtain a phase shift of approximately π or2π, or on the order of approximately 100 microns. Thus, thermo-opticphase shifters 122 have a negligible insertion loss of approximately 0.1decibels for a π shift. Therefore, when compared to an approach that isbased entirely on electro-optic phase shifters, the hybrid demultiplexer114 of the present disclosure results in a very low insertion loss,e.g., on the order of 0.3 dB per mixing stage 120 as compared to atleast 8 dB when four electro-optic phase shifters, while providingsufficient bandwidth.

FIG. 4 illustrates an example of a method utilizing the hybridpolarization demultiplexer 114 of the present disclosure. A light signalis received over a fiber element, at step 136. The light signal includespolarized optical signals. However, during transmission, these opticalsignals can become rotated and/or mixed. For example, a light signalcontaining orthogonally polarized optical signals X and Y may arrive atpolarization splitter 128. Polarization splitter 128 can separate thesignal components into X′ and Y′ of the light signal, at step 138.However, signals X′ and Y′ are only a combination of theoriginally-transmitted X and Y signal components due to the mixing thatoccurred during transmission along the fiber element. Demultiplexer 114includes a series of mixing stages 120 which are provided in a cascadedmanner, at step 140. Each mixing stage 120 includes a thermo-optic phaseshifter 122 and an electro-optic phase shifter 124. Feedback controllogic 130 provides separate control signals to the thermo-optic andelectro-optic phase shifters, at step 142. After the signals have passedthrough the mixing stages 120, demultiplexer 114 outputs the originallytransmitted polarized optical signals X and Y, at step 144.

The hybrid demultiplexer of the present disclosure provides acombination of electro-optic and thermal-optic phase shift elements suchthat each phase shift element is separately controlled: a dither signalcontrols the electro-optic phase shifter and a separate demultiplexercontrol signal controls the thermo-optic phase shifter. This allows theuse of dither tones having a frequency far above the control signalbandwidth. Advantageously, the hybrid demultiplexer of the presentdisclosure is applicable to silicon photonics networks because itutilizes only existing Silicon building blocks such as thermo-opticphase shifters, electro-optic phase shifters such assilicon-insulator-silicon capacitive (SISCAP) structures, and 2×2couplers.

The hybrid demultiplexer of the present disclosure also provides lowexcess loss which is superior to an electro-optic only approach, and lowpower consumption. For example, each thermal phase shifter consumesapproximately 25 mW while each electro-optic phase shifter consumesapproximately less than 10 mW. Thus, in the four phase shifter scenarioshown in the figures, less than 200 mW are consumed. Further, the hybriddemultiplexer described herein is compatible with pre-existing controlalgorithms used with other polarization demultiplexing methodologies,can be implemented without the need for coherent receivers or digitalsignal processors (DSPs), and can be implemented regardless of the typeof modulation format and data rate.

It is understood that any specific order or hierarchy of steps in theprocesses disclosed is an illustration of exemplary approaches. Basedupon design preferences, it is understood that the specific order orhierarchy of steps in the processes may be rearranged, or that only aportion of the illustrated steps be performed. Some of the steps may beperformed simultaneously. For example, in certain circumstances,multitasking and parallel processing may be advantageous. Moreover, theseparation of various system components in the examples described aboveshould not be understood as requiring such separation in all examples,and it should be understood that the described program components andsystems can generally be integrated together in a single softwareproduct or packaged into multiple software products.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but are to be accorded the full scope consistentwith the language claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.”

A phrase such as an “aspect” does not imply that such aspect isessential to the subject technology or that such aspect applies to allconfigurations of the subject technology. A disclosure relating to anaspect may apply to all configurations, or one or more configurations. Aphrase such as an aspect may refer to one or more aspects and viceversa. A phrase such as a “configuration” does not imply that suchconfiguration is essential to the subject technology or that suchconfiguration applies to all configurations of the subject technology. Adisclosure relating to a configuration may apply to all configurations,or one or more configurations. A phrase such as a configuration mayrefer to one or more configurations and vice versa.

The word “exemplary” is used herein to mean “serving as an example orillustration.” Any aspect or design described herein as “exemplary” isnot necessarily to be construed as preferred or advantageous over otheraspects or designs.

The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. It will, however, beevident that various modifications and changes may be made thereuntowithout departing from the broader spirit and scope of various aspectsof the disclosure as set forth in the claims.

We claim:
 1. A polarization demultiplexer comprising: at least one ahybrid phase shifter configured to receive a light signal over a fiberelement, the light signal including polarized optical signals, the atleast one hybrid phase shifter each comprising: a thermo-optic phaseshifter configured to phase shift the polarized optical signals; anelectro-optic phase shifter configured to phase shift the polarizedoptical signals; and control circuitry configured to regulate thethermo-optic and electro-optic phase shifters to reduce an insertionloss of the hybrid phase shifter while maintaining a sufficient controlbandwidth, the control circuitry further configured to send a controlsignal to the thermo-optic phase shifter and a dither signal to theelectro-optic phase shifter.
 2. The polarization demultiplexer of claim1, each hybrid phase shifter further comprising a coupler configured tomaintain polarization of the polarized signal components, the couplerand the hybrid phase shifter forming a mixing stage.
 3. The polarizationdemultiplexer of claim 2, further comprising: a plurality of cascadedmixing stages.
 4. The polarization demultiplexer of claim 1, wherein thecontrol signal and the dither signal having different amplitudes.
 5. Thepolarization demultiplexer of claim 4, wherein the dither signal has anamplitude that is less than an amplitude of the control signal.
 6. Thepolarization demultiplexer of claim 1, wherein the polarized signalcomponents are substantially orthogonal with respect to each other. 7.The polarization demultiplexer of claim 1, wherein the electro-opticalphase shifter includes one of a MOS capacitor, a PN junction, a P-I-Njunction, or a Lithium Niobate (LiNbO3) phase modulator.
 8. A hybridphase shifter comprising: a thermo-optic phase shifter configured tophase shift polarized optical signals of a light signal, thethermo-optic phase shifter receiving a control signal from controlcircuitry; and an electro-optic phase shifter configured to phase shiftthe polarized optical signals, the electro-optic phase shifter receivinga dither signal from the control circuitry, the control signal and thedither signal having different amplitudes.
 9. The hybrid phase shifterof claim 8, wherein the dither signal has an amplitude that is less thanan amplitude of the control signal.
 10. The hybrid phase shifter ofclaim 8, wherein the polarized signal components are substantiallyorthogonal with respect to each other.
 11. The hybrid phase shifter ofclaim 8, wherein the electro-optical phase shifter includes one of a MOScapacitor, a PN junction, a P-I-N junction, or a Lithium Niobate(LiNbO3) phase modulator.
 12. A method of demultiplexing apolarization-multiplexed optical signal, the method comprising:receiving a light signal over a fiber element, the light signalincluding polarized optical signals; and separating the polarizedoptical signals of the light signal received over the fiber element, theseparating comprising: providing a plurality of mixing stages, eachmixing stage comprising a thermo-optic phase shifter and anelectro-optic phase shifter adapted to phase shift the polarized opticalsignals.
 13. The method of claim 12, further comprising: applyingcontrol signals to the thermo-optic phase shifter and the electro-opticphase shifter of each mixing stage.
 14. The method of claim 13, whereinthe control signal applied to the thermo-optic phase shifter is separatefrom the control signal applied to the electro-optic phase shifter. 15.The method of claim 14, wherein the control signal applied to theelectro-optic phase shifter is a dither signal.
 16. The method of claim15, wherein the dither signal applied to the electro-optic phase shifterhas an amplitude different from an amplitude of the control signalapplied to the thermo-optic phase shifter.
 17. The method of claim 16,wherein the amplitude of the dither signal is smaller than the amplitudeof the control signal applied to the thermo-optic phase shifter.
 18. Themethod of claim 12, wherein the polarized optical signals aresubstantially orthogonal with respect to each other.
 19. The method ofclaim 12, wherein a phase shift provided by the electro-optic phaseshifter is shorter than a phase shift provided by the thermal-opticphase shifter.
 20. The method of claim 12, wherein the electro-opticalphase shifter includes one of a MOS capacitor, a PN junction, a P-I-Njunction, or a Lithium Niobate (LiNbO3) phase modulator.